U.S. Pat. No. 3,946,298 showed a spindle positioning system for a multiple spindle automatic machine tool wherein the workpiece spindle was rotated at a relatively slow speed by a servo-motor separate from the main motor normally driving the spindle for workpiece machining. A counter circuit counted pulses to angularly position the spindle relative to a reference angle. Such spindle positioner required considerable additional mechanism and hence was relatively expensive. Additionally, when spindles in two different spindle positions in a multiple spindle machine required positioning, then two separate servo-motors and control systems were required.
U.S. Pat. No. 3,643,307 indicates another prior art arrangement for positioning a spindle of a multiple spindle machine tool. A clutch and a brake are provided for each spindle between the main motor 95 and the spindle. A separator motor 103 is provided to slowly rotate the spindle to a desired position at which a counter 111 counts to zero. This is also a cumbersome arrangement and must be duplicated if spindle positioning at a different work station of the spindle carrier is desired.
The problem to be solved is how to accomplish a relatively slow-speed low-torque rotation of a spindle in a machine tool and further how the spindle may be accurately angularly positioned.